The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with Fourier transform or spin-warp imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with echo-planar imaging, echo-volume imaging, and other imaging and spectroscopy techniques in which only a partial or incomplete data set is available.
Conventionally, magnetic resonance imaging has included the sequential pulsing of radio frequency signals and magnetic field gradients across a region to be imaged. A patient is disposed in a region of interest in a substantially uniform main magnetic field. In two-dimensional imaging, an RF excitation pulse is applied as a slice select gradient is applied across the field to select a slice or other region of the patient to be imaged. A phase encode gradient is applied along one of the axes of the selected slice to encode material with a selected phase encoding. In each repetition of the pulse sequence, the phase encode gradient is stepped in regular intervals from a negative maximum phase encode gradient through a zero phase encode gradient to a positive maximum phase encode gradient. In three-dimensional volume imaging, a pair of phase encode gradients are applied along the two axes orthogonal to the read direction.
Because data lines on either side of the zero phase encode data lines are related to each other by conjugate symmetry, data acquisition time has been reduced by nearly one-half. In single slice imaging, one half of the data is collected, e.g., only the positive phase encode views or only the negative phase encode views, along with several lines adjacent to and on either side of the zero phase encode lines. The data lines near the zero phase encode data lines are used to generate a phase map for aligning the conjugately symmetric data. In three-dimensional imaging, the same concept is projected into the third dimension. A rectangle prism or thin slab of data is collected at the center of k-space to construct a phase map. Half of the remaining data is actually collected and conjugately symmetric data is used as the other half of the data.
Although faster than collecting all of the data lines, faster data acquisition times are in demand. The present invention contemplates a new and improved magnetic resonance imaging method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging includes exciting magnetic resonance in an imaging region and causing a magnetic resonance echo during which a magnetic resonance echo signal is generated. A plurality of phase encode gradients are applied such that the resultant magnetic resonance echo signals are phase encoded in accordance with the phase encode gradients. A non-rectangular central portion of the magnetic resonance echo signal surrounding a central frequency is collected along with a non-rectangular side portion of the magnetic resonance signals between the central portion and one extreme of a resonance signal bandwidth. A phase correction data value set is generated from at least a portion of the central and side portion data value sets. In addition, a conjugately symmetric data value set is generated from at least a portion of the central and side portion data value sets. The conjugately symmetric data value set is phase corrected in accordance with the phase correction data value set. The central portion, side portion, and conjugately symmetric data value sets are combined to produce a combined data set which is transformed into an intermediate image representation.
In accordance with a more limited aspect of the present invention, the magnetic resonance imaging method includes either exporting the intermediate image representation to a human-viewable display or using the intermediate image representation in a further iteration to generate a second conjugately symmetric data set.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging includes exciting magnetic resonance in an image region and inducing a magnetic resonance echo which generates magnetic resonance signals. The echo is phase and frequency encoded along an ellipsoidal central portion of k-space and along half of a peripheral region. An ellipsoidal central portion of k-space is sampled to create a central data set and a peripheral portion of k-space is sampled to create a peripheral data set. A phase correction data set is created from data values of at least one of the central data set and the peripheral data set. A conjugately symmetric data set is generated from at least one of the central and peripheral data sets. The conjugately symmetric data set is phase corrected in accordance with the phase correction data set. The central, peripheral, and conjugately symmetric data sets are combined to produce a combined data set, which is transformed to produce an image representation.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging includes generating magnetic resonance data including a non-rectangular first set of centrally encoded data values and a non-rectangular second set of data values which includes less than one half of the remaining data values. A phase correction data set is created from data values of at least one of the first and second sets of data values. A conjugately symmetric third data set is generated from the second data set. The first, second, and third data value sets are Fourier transformed and phase corrected in accordance with the phase correction data set. The phase corrected, Fourier transformed data sets are combined to produce an image representation.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging includes sampling a non-rectangular central region of k-space to generate a kernel data set and half-sampling a non-rectangular peripheral regions of k-space, with corners of k-space remaining unsampled, to generate an actually sampled peripheral data set. A symmetric data set is created from the actually sampled peripheral data set and the unsampled corners of k-space are zero filled. The kernel, actually sampled, and symmetric data sets are reconstructed into an image representation.
In accordance with another aspect of the present invention, a magnetic resonance imaging apparatus includes a magnetic resonance data means for generating a first non-rectangular set of centrally encoded data values and a second non-rectangular set of data values which includes less than one-half of the remaining data values. A phase correction generating means generates a phase correction value set from at least a portion of the first and second non-rectangular data value sets. A conjugate symmetry means generates a third non-rectangular data set from complex conjugate values of at least a portion of the first and second non-rectangular data value sets. A phase correcting means phase corrects the third non-rectangular data set in accordance with the phase correction value set. A combining means combines the first, second, and third non-rectangular data value sets to form a combined data set. A transform means transforms the combined data set into a resultant image representation.
One advantage of the present invention resides in reduced scan time.
Another advantage of the present invention is that it produces greater signal-to-noise ratio in the central portion of the scan.
Another advantage of the present invention resides in the elimination of susceptibility artifacts.
Another advantage of the present invention resides in the creation of a phase map focused in the region of primary interest.
Still another advantage of the present invention is that it applies phase conjugate symmetry in more than one dimension.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.